Light-emitting diodes (LEDs) are increasingly being utilized as discrete light sources in various illumination devices. As with illumination devices based on other light sources (e.g., incandescent bulbs), lenses or other types of optics may be utilized to collimate and direct the light from the LEDs. When a small individual LED die is utilized in an illumination device, and when the exit area of the optic is large compared to the area of the die, the LED die effectively acts as a point source of light, and conventional optics, such as a parabolic reflector (e.g., a parabolically curved metal surface) or convex lens, may be utilized—with the LED placed at the focus of the optic—to form a collimated beam of light for general-illumination applications such as utility lights or flashlights.
However, to increase the light output from LED-based illumination devices, multiple LED dies are often used in a single device, e.g., arranged in an array, to form an extended light source. This entails two problems: First, as the total area of the light source increases in comparison to the exit area of the optic, the light source is no longer a point source, and collimation of the light becomes difficult. For example, whereas a point source placed at the focus of a parabolic reflector produces a collimated beam, an extended light source placed at the focal plane inevitably results in some degree of beam divergence. Second, for practical reasons (such as manufacturing limitations), an extended light source composed of multiple discrete light sources rarely forms a continuous, uniformly emitting surface. In an array of LED dies, for example, the individual dies are typically spaced apart, resulting in dark borders between the rows and columns. As the light is collimated (or nearly collimated), these dark borders appear in the beam profile, disrupting its uniformity.
For most general illumination applications, such as to highlight an article (for example, in a retail store) or to throw the light beam a long distance to increase visibility of distant objects (e.g., as with a flashlight), it is desirable to have a narrow (i.e., low-divergence) beam of light free of visual artifacts and with a high central beam intensity. The desire for such artifact-free beams is often at odds with the drive to produce high-brightness beams with multiple LED dies. Various approaches have been used to minimize the imaging (i.e., in-beam visibility) of individual dies in order to achieve bright, yet uniform beams. In general, however, the methods used to minimize the visibility of individual dies in an illumination device that contains many dies often conflict with the methods utilized to produce a narrow optical beam and the need to preserve high optical efficiency.
In one approach, features placed at the exit surface of the optic are used to disperse the beam, i.e., direct light away from the central axis. For example, the surface can be textured at small scales or incorporate periodic shapes or elements (such as arcs) that diverge the beam. (Such exit-surface dispersion optics are often called “pillow optics” since they consist of an array of curved surfaces resembling pillows.) The overlap between individual divergent beams resulting from the light-dispersing features may achieve a relatively uniform illuminated surface, but also causes undesirable broadening of the overall light beam. Likewise, more finely divided surfaces closer to the wavelength of the light can cause light scattering, again resulting in a divergent beam that obscures visual artifacts but is undesirably broadened. Furthermore, because the finely divided surface is easily damaged by contact and can be easily modified by contaminants that change the interfacial optical properties, exit-surface light dispersion is unsuitable for many applications.
In another approach, which is often used in traditional reflectors, the surface of the optic is faceted, i.e., approximated by multiple (typically planar) surface segments. Faceting moves the light slightly off the central axis. By faceting over the entire surface of the reflector, imaging of the light source may be prevented—but once again at the expense of broadening the light beam. Hence, faceting is often used in floodlights. In general, the larger the facets, the more obscured the non-uniformities of the light-emitting surface will be. However, larger facets themselves introduce artifacts in the light beam, thus undermining the beam uniformity they are intended to accomplish.
Accordingly, there is a need for alternative optics for illumination devices incorporating LED dies as their light sources that reduce non-uniformities resulting from the arrangement of the light sources while minimizing beam broadening and artifacts introduced by the optic itself.